The present invention relates to a hydraulic control system for a hydrokinetic torque converter including a lockup clutch.
As is well known in the art, in a hydrokinetic torque converter there is a slippage between a pump impeller and a turbine runner during operations of the torque converter. For this reason, although it is easy to operate and manipulate a vehicle equipped with an automatic transmission including the torque converter as a part of a power transmitting train, the fuel consumption of the vehicle is high. As a solution to this problem, various hydrokinetic torque converters have been proposed which lock-up when the vehicle operates at high speeds in high gear.
A lock-up control valve is provided, in a known torque converter, which pressurizes a lock-up chamber for releasing a lock-up condition or exhausts it for effecting the lock-up condition.
In order to effect the lock-up condition of the torque converter in response not only to gear position or positions of the automatic transmission but also to vehicle speed, it has been proposed by the same inventors of the present application to arrange a cut-off valve, between the control valve and a passageway which is pressurized with line pressure when the highest gear is selected, for allowing the line pressure to act on the control valve in response to governor pressure higher than a predetermined level. The control valve will exhaust the lock-up chamber upon receiving the line pressure from the cut-off valve.
A problem with this control system resides in that the provision of the cut-off valve, in addition to the control valve, results in an increase in complexity of construction of the control system and in an increase in the number of steps in piping of the control system, thus boosting up the cost of manufacturing automatic transmissions.